Mine roof expansion shell assembly

ABSTRACT

A mine roof expansion shell assembly including two identical and cooperating shell segments, each having a base portion and an elongated wall portion which extends from the base portion and which has an increasing thickness in a direction away from the base portion. The base portion of each shell segment has a projection extending outwardly therefrom laterally of the shell segment and a recess therein which extends inwardly of the base portion, also in a direction laterally of the shell segment. The projection and recess are adapted to mate with the recess and the projection in the base portion of the cooperating shell segment, when the shell segments are arranged to form the shell assembly, and in such a way that the projections and recesses cooperate to prevent axial movement of one shell segment relative to the other shell segment.

United States Patent Pete [54] MINE ROOF EXPANSION SHELL ASSEMBLY [72] lnventor: George H. Pete, 2519 W. Macon, Decatur, 111. 62525 [22] Filed: April 6, 1970 [21] App1.No.: 25,987

[52] [1.5. CI. ..85/88, 61/45 B [51] Int. Cl ..Fl6b 13/04 [58] Field of Search ..61/45 B; 85/67, 88, 66, 68, 85/69; 52/704, 698

[56] References Cited UNITED STATES PATENTS 1,116,545 11/1914 Barrett ..85/88 1,655,808 l/l928 Bodkin ..85/88 1,790,841 2/1931 Rosen ..85/66 3,065,659 1 1/1962 Eriksson et a1. ..85/67 3,226,934 l/l966 Emery ..61/45 B 925,006 6/1909 Mason ..85/67 2,241,343 5/1941 Fleaca ..61/45 B UX 2,783,673 3/1957 Lewis et a1. ..61/45 B X 2,988,950 6/1961 Dempsey ..61/45 B X 1 Aug. 15, 1972 FOREIGN PATENTS OR APPLICATIONS 80,883 5/1963 France ..61/45 B 1,107,170 5/1961 Germany ..61/45 B Primary Examiner-Dennis L. Taylor Attorney-Greist, Lockwood, Greenawalt and Dewey [5 7] ABSTRACT A mine roof expansion shell assembly including two identical and cooperating shell segments, each having a base portion and an elongated wall portion which extends from the base portion and which has an increasing thickness in a direction away from the base portion. The base portion of each shell segment has a projection extending outwardly therefrom laterally of the shell segment and a recess therein which extends inwardly of the base portion, also in a direction laterally of the shell segment. The projection and recess are adapted to mate with the recess and the projection in the base portion of the cooperating shell segment, when the shell segments are arranged to form the shell assembly, and in such a way that the projections and recesses cooperate to prevent axial movement of one shell segment relative to the other shell segment.

4 Claims, 6 Drawing Figures P'A'TE'NTEDws 15 I972 INVENTOR. GEORGE H.PETE 3%;), JMM

FIG! *h' ATTORNEYS.

MINE RUOF EXPANSION SHELL ASSEMBLY The present invention relates to a mine roof bolt assembly and more particularly to an expansion shell assembly used in the bolt assembly and formed from identical and cooperating shell segments.

A mine roof bolt assembly including an expansion shell assembly, a nut or threaded wedge member positioned within the shell assembly, and a bolt threadably received in the nut or wedge member, for securing a washer plate or other fixture to the roof of a mine, is well known. In this respect, a variety of expansion shells and shell assemblies have been proposed.

One such known expansion shell is of one piece construction. This type of expansion shell has the disadvantage that the side wall portions thereof can flex laterally of the shell assembly only a small distance, if they can flex at all. As a result, the wedging action which can be obtained with this type of expansion shell is-limited. Additionally, it is more difficult to die cast a one piece shell than a shell segment or shell half.

Thus, it was long ago proposed to utilize a shell assembly formed from at least two shell segments or shell halves. Also, it has been proposed to use a standard nut such as a hex nut in a shell assembly defined by two identical shell halves. However, this type of shell as sembly formed from two identical shell halves has the disadvantage or problem of movement of one shell half in a longitudinal direction relative to the other shell half. When this occurs, undue stress is placed upon one shell half and in many instances, an inadequate and insufficient wedging of the side wall portions of the shell halves into the side wall of a bore in a mine roof, is obtained.

In view of these difficulties incurred with the formation of a shell assembly from two identical shell halves, it was long ago proposed to connect one end (or both ends) of one shell half to the corresponding end of the cooperating shell half with a flexible or frangible member. However, this type of shell assembly utilizing a frangible or flexible connecting member, such as a flexible connecting member made of a spring metal, increased the cost of the shell assembly. In other words, the basic cost of the shell assembly, e.g., the cost of casting each shell half or segment, was increased by the additional cost of the spring metal connecting member and the cost of connecting the spring metal connecting member to corresponding ends of the shell segments.

The present invention overcomes many of these deficiencies incurred with prior shell assemblies by providing a shell segment having a base portion and an elongated wall portion extending from the base portion and wherein a recess extends into the base portion and a projection extends from the base portion for cooperating or mating with similar formations on an identical shell segment or segments to form a shell assembly in which movement of one shell segment longitudinally of the shell assembly is prevented, while outward movement of the wall portions of each shell segment into engagement with the side wall of a bore is permitted, without the use of a flexible or frangible metal connecting member.

Accordingly, an important object of the present invention is to provide an expansion shell assembly formed from identical and cooperating shell segments, each of which has a projection extending from, and a recess extending into the base portion of the shell segment, the projection and the recess being adapted to engage similar fonnations on the other shell segment or segments to prevent movement of one shell segment longitudinally of the shell assembly.

Another object of the present invention is to provide a mine roof expansion shell assembly formed of shell segments which are easy to sand cast.

Another object of the present invention is to provide a mine roof expansion shell assembly which is adapted to utilize a standard square nut.

Another object of the present invention is to provide a shell segment which is particularly adapted for use in an expansion shell assembly and which has a recess extending into the base portion laterally of the shell segment and a projection extending outwardly from the base portion laterally of the shell segment, the shell seg ment being adapted to cooperate with at least one other shell segment to form the shell assembly with the projection one one shell segment being received in the recess in another shell segment and the projection being smaller than the recess to permit limited movement of the projection transversely of the recess.

Another object of the present invention is to provide a mine roof expansion shell assembly which is adapted to utilize a standard square nut and which is formed from two cooperating and identical shell segments which are inter-locked against movement longitudinally of the shell assembly by the cooperation of inter-engagement of a recess in and a projection on one segment with a projection on and recess in the cooperating shell segment.

Another object of the present invention is to provide a mine roof expansion shell assembly formed from two cooperating and. identical shell segments which are inter-locked against movement longitudinally of the shell assembly by the cooperation of a recess in and a projection on each shell segment with a projection on and a recess in the cooperating shell segment, the assembly being adapted to utilize a standard square nut, and including a tape or band around the two shell segments for holding the shell assembly together prior to the insertion of same into the bore in a mine roof.

Another object of the present invention is to provide a shell segment which is adapted for use in a mine roof expansion shell assembly and which includes a base portion and an elongated side wall portion with the base portion having a recess therein extending in a direction laterally of the shell segment and a projection thereon extending laterally outwardly from the shell segment.

Another object of the present invention is to provide a shell segment which is adapted for use in a mine roof expansion shell assembly and which includes a base portion and an elongated side wall portion with the base portion having a recess therein extending in a direction laterally of the shell segment and a projection thereon extending laterally outwardly from the shell segment and wherein the projection has first and second laterally extending oppositely facing surfaces which face longitudinally of the shell segment and the recess has first and second laterally extending surfaces which face toward each other and in a direction longitudinally of the shell segment, the surfaces on the projection and in the recess providing bearing surfaces against which similar surfaces in the recess and on the projection of a cooperating shell segment will bear when one of the shell segments is urged in a direction longitudinally of the shell segmenes.

Another object of the present invention is to provide a shell segment which is adapted for use in a mine roof expansion shell assembly and which includes a base portion and an elongated side wall portion with the base portion having a recess therein extending in a direction laterally of the shell segment and a projection thereon extending laterally outwardly from the shell segment and wherein the side wall portion has at least two inner planar surfaces lying in respective planes which intersect each other and the planar surfaces are adapted to engage side surfaces of a nut to prevent turning of the nut relative to the shell segment.

Another object of the present invention is to provide a shell segment which is adapted for use in a mine roof expansion shell assembly and which includes a base portion and an elongated side wall portion with the base portion having a recess therein extending in a direction laterally of the shell segment and a projection thereon extending laterally outwardly from the shell segment and wherein the side wall portion includes first and second spaced apart legs which extend from the base portion longitudinally of the shell segment, each of the legs having'an inner planar surface lying in a plane which intersects the plane of the inner planar surface on the other leg at an angle approaching 90 whereby, when two cooperating shell segments are arranged to form a shell assembly, the inner planar surfaces will engage the side surfaces of a square nut to hold the nut against rotation while allowing movement of the nut along and longitudinally of the inner planar surfaces.

Another object of the present invention is to provide a shell segment which is adapted for use in a mine roof expansion shell assembly and which includes a base portion and an elongated side wall portion with the base portion having a recess therein extending in a direction laterally of the shell segment and a projection thereon extending laterally outwardly from the shell segment and wherein the side wall portion includes first and second spaced apart legs which extend from the base portion longitudinally of the shell segment, each of the legs having a curved outer surface with gripping means thereon in the form of serration, and the serrations on each of the legs including edges which extend in a direction longitudinally of the shell segment and in a direction laterally of the shell segment with the edges on one leg defining portions of lines which intersect the lines defined by the edges on the other legs so that the edges of the serrations will engage and dig into the side wall of a bore in a mine roof in a manner which prevents axial as well as rotational movement of the shell segment relative to the bore.

Still another object of the present invention is to provide a shell segment which is adapted to cooperate with one or more identical shell segments to form an expansion shell assembly and which includes a base portion and an elongated wall portion extending longitudinally from the base portion, the base portion having a recess therein and a projection thereon which are adapted to mate with cooperating formations on the other segment or segments of the shell assembly to prevent longitudinal movement of the shell segment relative to the shell assembly and wherein each side wall portion has at least one planar surface lying in a plane which will intersect a line extending longitudinally along and lying on the outer surface of the side wall portion, at an angle of between 3 and 7.

These and other objects of the invention, and the manner of their attainment, will become more apparent from the following detailed description of one preferred embodiment of the invention taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a fragmentary vertical sectional view, with portions broken away, of the mine roof bolt assembly of the present invention positioned within a bore in a mine roof;

FIG. 2 is a top plan view taken along line 2-2 of FIG. 1 and showing the mine roof expansion shell assembly of the present invention with a square nut received therein;

FIG. 3 is a vertical elevational view of the shell assembly shown in FIG. 2 taken along line 33 of FIG. 2;

FIG. 4 is a vertical elevational view of one of the shell segments of the shell assembly shown in FIG. 3;

FIG. 5 is a vertical elevational view of the interior of a shell segment shown in FIG. 4 taken along line 5-5 of FIG. 4;

FIG. 6 is a bottom plan view of the shell segment shown in FIG. 5 taken along line 6-6 of FIG. 5.

Referring now to the drawings in greater detail, a mine roof bolt assembly is generally indicated at 10 in FIG. 1 and is shown positioned within a bore 12 formed in a mine roof. The mine roof bolt assembly 10 includes a mine roof expansion shell assembly 14, which is adapted to engage the side wall 15 of the bore 12, a nut 16 which is received within the shell assembly 14, a bolt 18 which has a threaded end 19 threadingly received in the nut 16 and a polygonal head portion 21, and a washer plate or other fixture 22 which is secured to the roof of the mine by turning the bolt head 21 to cause the nut 16 to move axially of the bolt 18 and force the shell assembly into engagement with the side wall 15 of the bore 12.

In the illustrated embodiment, the shell assembly 14 includes first and second cooperating and identical shell segments 26 and 28 and, since the shell segments 26 and 28 are identical, only the shell segment 26 will be described in detail in connection with the description of FIGS. 1-6.

As best shown in FIGS. 1 and 5, the shell segment 26 includes a base portion 30 and an elongated side wall portion 32 which extends from the base portion 30 in a direction longitudinally of the shell segment 26. As shown in FIG. 5, the side wall portion 32 includes first and second, spaced apart legs 33 and 34 which are fixed to (integral with) the base portion 30 at one end, and are free at the other or distal end, 35 and 36 respectively. Each leg 33 and 34 is tapered in a direction longitudinally of and outwardly toward the free end 35 or 36 thereof. Stated otherwise, each leg 33 and 34 increases in thickness in a direction outwardly from the base portion 30. The leg 33 has a curved outer surface 38 defining part of a cylinder and an inner planar surface 40. Referring to FIG. 1, the inner planar surface 40 lies in a plane which intersects a line, lying on the outer surface 38 and extending in a direction longitudinally of the shell assembly 14, at an angle A.

The leg 34 is a mirror image of the leg 33 and has a curved outer surface 42 similar to the curved outer surface 38 and an inner planar surface 44 similar to the inner planar surface 40.

Preferably, the angle A is between 3 and 7 to provide a sufficient wedging action of the legs 33 and 34 outwardly from the axis of the bolt 18 with a minimum thickness of the legs 33 and 34 while retaining sufficient strength in the legs 33 and 34, particularly at the beginning of their taper adjacent the base portion 30.

As best shown in FIG. 2, the inner planar surface 40 lies in a plane which intersects the plane including the inner planar surface 44 at an angle approaching 90. In this way, when the shell segments 26 and 28 are arranged to form the shell assembly 14 as shown in FIGS. 1-3, the planar surfaces 40 and 44 on each shell segment 26 and 28 are positioned to engage respective sides of the square nut 16, to hold the square nut 16 against rotation while permitting the square nut 16 to move longitudinally of the shell assembly 14 as the threaded portion 19 of the bolt 18 is rotated to wedge the legs 33 and 34 of each shall segment 26 and 28 outwardly into engagement with the side wall of the bore 12. In this respect, the side wall portions 32 of the shell segments 26 and 28 are forced laterally outwardly of the shell assembly 14 in the direction indicated by the arrows 49 and 50in FIG. 3.

As best shown in FIGS. 3 and 4, each of the legs 33 and 34 is provided with serrations 52,54 on the curved outer surface 38, 42 thereof. These serrations 52 and 54 define a means for gripping and digging into the side wall 15 of the bore 12 when the legs 33 and 34 are wedged into the side wall 15 by the wedging action of the nut 16 as it is moved along the inner planar surfaces 40 and 44. The serrations 52 and 54 are formed by edges 56 and 58 which extend in a direction longitudinally and laterally of each shell segment 26 and 28, the edges 56 defining lines which intersect lines defined by the edges 58 so that the edges 56 and 58 will dig into the side wall 24 of the bore 12 in a manner which prevents rotational movement of the shell segment 26 as well as axial movement of the shell segment 26 relative to the side wall 15 of the bore 12.

As shown in FIG. 3, the shell assembly 14 can also include a tape or band 60 which surrounds the shell assembly 14 to hold the shell segments 26 and 28 together. If desired, the tape or band 60 can be removed prior to the insertion of the shell assembly 14 into the bore 12, or can be left on the shell assembly 14 when the shell assembly 14 is inserted in the bore 12. In FIG. 1, the band 60 has been removed.

In order to permit some relative movement of the shell segment 26 relative to the shell segment 28 and in order to minimize the cost of the shell assembly 14, the band 60 is preferably formed from an inexpensive elastic material such as a rubber, a synthetic rubber or a resilient plastic material.

I As best shown in FIGS. 1 and 5, the base portion 30 has a longitudinally facing end surface 62 and an oppositely and longitudinally facing inner surface 64. As shown in FIG. 6 the inner surface 64 is generally trian gular in extent so that the inner surfaces 64 of the shell segments 26 and 28 together define a generally square seating surface against which the nut 16 can rest prior to the end portion 19 of the bolt 18 into the nut 16. As

shown in FIG. 2, the end surface 62 is generally semicircular.

The base portion 30 has a predetermined thickness between the surfaces 62 and 64 and a generally semicircular transverse cross section. Extending between.

the two longitudinally facing surfaces 62 and 64 are two side surfaces 66 and 68. The side surface 66 is curved and, in the illustrated embodiment, defines part of a cylinder, whereas the side surface 68 is a planar surface which extends longitudinally of the shell segment 26 and which faces in a direction laterally of the shell segment 26.

As best shown in FIGS. 4-6, the base portion 30 has a projection 70 which extends outwardly from the planar surface 68. In the illustrated embodiment, the projection 70 extends generally perpendicular to the planar surface 68. The projection 70 has a first side surface 72, which is curved and which forms a continuation of the curved surface 66, and a second side surface 74 which lies in a plane which extends longitudinally of the shell segment 26 and in a direction transversely of, and perpendicular to, theplanar surface 68. As shown, the end 76 of the projection 70 is rounded and connects the curved side surface 72 with the planar side surface 74. Additionally, the projection 70 has first and second oppositely facing laterally extending surfaces 77 and 79 which face longitudinally of the shell segment 26. The thickness of the projection between the surfaces 77 and 79 is less than the thickness of the base portion 30 between the surfaces 62 and 64.

Additionally, the base portion 30 has a recess 80 formed therein, as best shown in FIG. 5. It will be understood, that the projection 70 on the shell segment 26 is received in the recess 80 in the-shell segment 28 to prevent movement of the shell segment 26 longitudinally of the shell segment 28 and vice versa.

As shown in FIGS. 4, 5 and 6, the recess 80 is in the form of a generally U-shaped slot which opens onto the planar surface 68 and extends from the planar surface 68 into the base portion 30. As shown, the slot or recess 80 extends transversely of and perpendicular to the planar surface 68 and, as best shown in FIGS. 3 and 4, the recess 80 also opens onto the curved surface 66 of the base portion 30. V

The U-shaped recess or slot 80 is defined by an inner surface 81, lying in a plane which extends longitudinally of the shell segment 26 and in a direction transversely perpendicular to the planar surface 68, and by first and second laterally extending surfaces 83 and 85 which face toward each other and in a direction l0ngitudinally of the shell segment 26. The width of the recess 80 between the surfaces 83 and 85 is greater than the thickness of the projection 70 between the surfaces 77 and 79, so that, when the projection 70 is received in the recess 80 (as shown in FIG. 3), limited movement of the projection 70 within the recess 80 in a direction longitudinally of the shell assembly 14 is permitted. In this respect, it is desired that some clearance or *play" be provided between the surfaces 83 and 85 of the recess 80 and the surfaces 77 and 79 of the projection 70 to allow the projection 70 to twist in the recess 80 about a horizontally extending axis which is parallel to the planar surface 68 when the side wall portions 32 are wedged outwardly in the direction indicated by the arrows 49 and 50..

Also, it will be noted that lateral or diametrical movement of one shell segment 26 relative to the other shell segment 28 is limited by the engagement of the side surface 74 of the projection 70 on one base portion 30 with the inner surface 81 of the recess 80 in the other base portion 30.

If desired, the shell segment 26-(and in like manner, the shell segment 28) can have at least one locating and spacing projection or boss 90 extending from the planar surface 68 of the base portion 30 at a point above the recess 80. The boss 90 on the shell segment 26 will engage the planar surface 68 on the shell segment 28 at a point above the projection 70 and vice versa. The boss 90 on each shell segment 26 or 28 will slide and/or pivot on the planar surface of the other shell segment 28 or 26 when the nut 16 is moved from a position adjacent the inner surfaces 64 of the shell segments 26 and 28 toward the free ends 35 and 36 of the legs 33 and 34 to cause wedging of the legs 33 and 34 outwardly in the direction of the arrows 49 and 50, as shown in FIG. 3. It will be appreciated, that the bosses 90 reduce the amount of frictional engagement between the base portions 30 of each shell segment 26 and 28 when the nut 16 is moved toward the free ends 35 and 36 of the legs 33 and 34.

If desired, each shell segment can include three bosses with the two additional bosses 91 and 92 extending from the planar surface 68 at positions shown by phantom lines in FIG. 5. The bosses 90-92 can be designed to extend from the planar surface the same distance or different distances for aligning and locating one shell segment 26 in a predetermined position relative to the other shell segment 28.

Although the shell segments 26 and 28 of the shell assembly 14 of the present invention are shown as shell halves, it is to be understood that a shell assembly 14 can be formed with more than two segments. For example, three segments having two planar surfaces can be utilized to form the shell assembly. A projection will extend from one of the planar surfaces'and a recess will extend from the other planar surface into the shell segment so that the segments can be arranged to form an assembly which is adapted to hold a standard hex nut as opposed to a standard square nut. In this modified embodiment of the shell assembly the transverse cross section of each base portion of each shell segment will be partially circular, e. g., a pie-shaped cross section, equal to one-third of a circle.

Also, it is to be understood that the wall portions 32 of each shell segment 26 and 28 can be formed with more than two inner planar surfaces 40 and 44 such that the shell assembly 14 can utilize a hex nut or other polygonal nut instead of a square nut. Additionally, the shell assembly 14 can utilize a wedge nut of a type commonly used in mine roof bolt assemblies.

Alternatively, to provide a larger wedging surface area against the inner planar surfaces 40 and 44 of each shell segment 26 and 28, the shell assembly 14 can include an apertured non-threaded four sided wedge member situated on the bolt 18 between the head 21 of the bolt 18 and the nut 16 and within the confines of the side walls 32 of the shell assembly 14.

In another modified embodiment of the shell assembly 14 the base portion 30 of each shell segment 26 and 28 has a half circular opening extending between the surfaces 62 and 64 and opening onto the planar surface 68 to define with the corresponding opening in the base portion 30 of the cooperating shell segment 28, a bore through which the threaded end 19 of the bolt 18 can extend.

It will be understood from the foregoing description that an expansion shell assembly formed from shell segments made in accordance with the teachings of the present invention, has a simple construction and is inexpensive to manufacture. In this respect, it will be understood that the casting of a shell segment is much easier than the casting of a whole shell. Moreover, by using identical shell segments to form the shell assembly 14 only one type of shell segment is needed. In other words, one does not have to mate a right hand shell half with a left hand shell half. Additionally, and most importantly, the cooperating projections and recesses provide a simple and inexpensive means for preventing longitudinal movement of one shell segment 26 relative to the other shell segment 28 when the shell segments 26 and 28 are arranged in cooperating relationship to form the shell assembly 14.

Thus, the present invention has numerous advantages and application, some of which have been described above and others of which are inherent in the invention. Therefore, the scope of the invention is only to be limited as required by the following claims.

I claim:

1. A shell segment for use in a mine roof expansion shell assembly formed by a second identical shell segment, a standard machine bolt and a standard nut, said shell segment including an upright generally semi-cylindrical base portion and two elongated finger-like side wall portions extending from said base portion and having exterior surfaces which lie generally in an extension of the curved side surface of said base portion, each of said side wall portions increasing in thickness from said base portion toward its distal end to provide a generally planar inner cam surface insecting chords of said extension of said curved side surface, each of said planar surfaces in combination with the other corresponding planar surfaces of said assembly being adapted to engage one side of said standard nut and thereby prevent turning of said nut within said assembly while providing a wedging action tending to separate said finger-like side wall portions at their distal ends, said semi-cylindrical base portion having a horizontal locking projection protruding from one of the opposite sides of its flat side wall surface at a location intermediate the opposite ends of said base portion, and said semi-cylindrical base portion also having a horizontal locking recess extending inwardly into the curved side thereof from the other of said opposite sides of said flat side wall surface at a location intermediate the opposite ends of said base portion, two of said shell segments mating together so that said horizontal locking projection on each segment projects into the horizontal locking recess of the mating segment whereby said mating segments are interlocked both axially and laterally.

2. The shell segment as defined in claim 20 wherein each of said inner planar surface lies in a plane which will intersect a longitudinally extending line lying on the outer surface of said side wall portion at an angle of between 3 and 7.

3. A shell segment as defined in claim 1 including at least one locating and spacing boss which extends from said planar surface of said base portion. 

1. A shell segment for use in a mine roof expansion shell assembly formed by a second identical shell segment, a standard machine bolt and a standard nut, said shell segment including an upright generally semi-cylindrical base portion and two elongated finger-like side wall portions extending from said base portion and having exterior surfaces which lie generally in an extension of the curved side surface of said base portion, each of said side wall portions increasing in thickness from said base portion toward its distal end to provide a generally planar inner cam surface insecting chords of said extension of said curved side surface, each of said planar surfaces in combination with the other corresponding planar surfaces of said assembly being adapted to engage one side of said standard nut and thereby prevent turning of said nut within said assembly while providing a wedging action tending to separate said finger-like side wall portions at their distal ends, said semi-cylindrical base portion having a horizontal locking projection protruding from one of the opposite sides of its flat side wall surface at a location intermediate the opposite ends of said base portion, and said semi-cyliNdrical base portion also having a horizontal locking recess extending inwardly into the curved side thereof from the other of said opposite sides of said flat side wall surface at a location intermediate the opposite ends of said base portion, two of said shell segments mating together so that said horizontal locking projection on each segment projects into the horizontal locking recess of the mating segment whereby said mating segments are interlocked both axially and laterally.
 2. The shell segment as defined in claim 20 wherein each of said inner planar surface lies in a plane which will intersect a longitudinally extending line lying on the outer surface of said side wall portion at an angle of between 3* and 7*.
 3. A shell segment as defined in claim 1 including at least one locating and spacing boss which extends from said planar surface of said base portion.
 4. The shell segment as defined in claim 1 including three locating and spacing bosses which extend from said planar surface of said base portion. 